1. Field of the Invention
This invention relates to methods and apparatus for providing jackets on cable.
2. Description of Related Art
In the manufacture of electrical cable, cable cores are surrounded by elastomeric jacketing material by extrusion techniques. There is a choice of jacketing materials dependent upon the design and use of the finished cable. For instance, for cables to be used inside buildings, jackets of polyvinylchloride compounds are normally required as these materials are considered to be more fire and flame resistant than other materials. For other uses, jackets formed from polyolefin compounds are used.
Problems are associated with the extrusion of jacketing materials. In particular, in the extrusion of polyolefin jacketing materials, e.g., compounds of polyethylene, in a known phenomenon, there is sometimes an interaction between surfaces of the extrusion apparatus and certain surface regions of the flowing extrudate which slows down the extrusion speed at these surface regions. The main extrudate continues to flow around these surface regions which may become stationary upon the surfaces of the extrusion apparatus, and further extrudate from the main flow stream may accumulate upon these stationary surface regions. This accumulation tends to continue until eventually it is caused, by the flow of the main stream, to break away from the surfaces of the extrusion apparatus and pass through the extrusion orifice, thereby deleteriously affecting the finished product. For instance, where a core tube is being used to guide a cable core through the extrusion apparatus, an accumulation of extrudate may form a partial or complete annular ring around the downstream end region of the core tube before that accumulation breaks away. As a result, this accumulation, which does not mix with the main extrudate flow, forms a circumferentially extending lump between the jacket and the cable core, thus creating an overlying ridge in the jacket. Such a ridge is unsightly and may be unacceptable to a customer, one reason possibly being because it locally increases the cable diameter and adds to problems in drawing the cable through holes which are just sufficiently large to accommodate the normal diameter of the cable.